NDIR gas analysis measures the concentration of a gas in a sample by determining the amount of absorption of light which occurs at wavelengths which are normally selected to coincide with a relatively strong absorption band that is characteristic of the gas to be measured. In its simplest form, an NDIR gas analyzer contains a radiation source, an optical interference filter, a sample chamber, a detector and associated electronics. In operation, light is emitted from the radiation source and passed through the sample chamber where a portion of the light is absorbed by a sample gas. Next, light is passed through the filter to remove undesired wavelengths of light and then the remaining filtered light is passed on to the detector which measures the strength of the filtered light. Finally, the associated electronics calculate the concentration of the gas being measured in the sample cell.
The theory of NDIR gas analysis is well established. It has long been considered one of the best methods for gas measurement. However, it is not suitable for many uses because of its complicated and expensive implementation. Over the years, various improvements have been made to simplify NDIR gas analyzers in order to reduce the cost of such devices. Examples of some improvements are set forth in U.S. Pat. Nos. 5,163,332, 5,222,389 and 5,340,986. While such improvements have advanced the state of the art of NDIR gas analyzers, there are still many applications in which NDIR gas analyzers cannot be used when low cost is an integral design constraint.
All known NDIR gas analyzers require an IR source and an IR detector pair in some form, fit and function. Even when low cost is an integral design constraint, other design constraints must also be met. Such constraints include reliability, durability, stability, long life, low power and high efficiency. An incandescent miniature light bulb utilized as an IR source meets many of these design constraints, although there are other types of resistive thermal radiation sources which can also be used as IR sources in NDIR gas analyzers, such as platinum resistive sources and NiCr filament based sources. In all such sources, the particles of light are emitted from the surface of the source due to increased temperature of the source which is achieved by passing electrical current through the electrically resistive body of the source. The intensity and spectral content of light is controlled by the temperature of the surface of the source, which is controlled by controlling the amount of electical power applied to the source.
An incandescent miniature light bulb is typically composed of a tungsten filament vacuum sealed within a glass envelope to prevent oxidation which would severely cut the life time of the tungsten filament. In operation, the light bulb is coupled to a power source in an electrical circuit. As an electrical current is passed through the bulb, the bulb will give off light. The tungsten filament in of itself is an electrically resistive material, and the bulb has an initial resistance which can be measured. However, over time, the resistance of the bulb can vary. As the resistance of the IR source changes over time, the amount of electrical power delivered to the source will not be constant in the case when a simple contant current or a constant voltage source is used to drive the bulb, and therefore the lamp temperature will change over time. Absent correction for such changes, an NDIR gas analyzer using such an IR source will lose its calibration over time. Some prior art IR sensor designs include an optical reference channel to compensate for changes in lamp brightness aging affects, with or without a true constant power source driver. Various software algorithms have also been developed to compensate for aging. However, these solutions are complicated and utilize feedback loops that might become unstable.
Accordingly, a need exists for an inexpensive improved power circuit for use with a resistive thermal radiation source which will remain stable over time. In addition, there is also a need for further improvements which will reduce the cost of NDIR gas analyzers and thereby increase the applications in which such devices can be used.